This invention relates to circuits for reducing distortion produced by an r.f. power amplifier, particularly but not exclusively for SSB (Single Side Band) transmitters.
When an r.f. signal is amplified, e.g. in a transmitter, any non-linearity in the amplifier will cause envelope amplitude and/or phase distortion of the output compared to the r.f. signal source (drive waveform). This will result in intermodulation distortion of the amplified r.f. signal.
It has been proposed (GB-A-1246209) to compensate for any distortion produced by an r.f. amplifier by separately correcting for envelope amplitude deviation and r.f. phase deviation between the power amplifier output and the drive waveform input. It is known (GB-A-1246209 and GB-A-1474952) to correct for envelope amplitude deviation by a feedback loop including envelope detector means for generating an error signal dependent on the difference between the envelope amplitudes of a signal derived from the power amplifier output and a signal derived from the r.f. signal source. The error signal modulates the envelope amplitude of the power amplifier input (which effectively alters the gain of the power amplifier) in such a manner as to reduce envelope distortion. Since the distortion component is extracted by assessing the difference between two envelope amplitudes, it follows that those amplitudes must be held the same in order that a signal representative of the rapid fluctuations between the two envelopes can be derived so that the distortion can be compensated for.
However, to detect the envelope amplitude of the power amplifier output requires a probe which will have a different amplitude response at different r.f. frequencies. Equally, the cable losses from the corrector unit to the transmitter output probe may vary with frequency. Consequently, to operate at different r.f. frequencies, adjustments must be made for each frequency to make the envelope amplitudes compared the same.